I never had any drop. After you kill the multiple Guardian dragons defending their roosts on the way to Dragon Shrine they're guarding several treasures. After killed you can go back through and you'll see that there are TONS of turtles (about 2-4 IIRC per roost) and each seemed to drop a bone and twinkling for me. So this ends up to be about 9 or so bones. You definitely need to be very conservative with your bones, but to be frank there's not really that many good boss items.
The dragon acolytes right before dragon shrine (can't remember the name of the area) drop them, not sure if they're rare, or if it's because I have my area in NG+, but farm them until they stop spawning, then NG+ it. Remember there's some hiding behind paintings in the hallway, I knocked one down accidentally, and really glad I found this out.